Surface modified calcium carbonate containing minerals and its use

ABSTRACT

The present invention is related to ball-shaped spherical surface modified spherical calcium carbonate comprising minerals, the process for preparing such ball-shaped spherical surface modified spherical calcium carbonate comprising minerals, and their use.

FIELD OF THE INVENTION

The present invention is related to calcium carbonate containing minerals, and in particular to ball-shaped spherical surface modified spherical calcium carbonate comprising mineral particles with a mean particle size diameter above 1 μm, and its use. A particular use is in paints and coatings, providing for a surface matting effect of the paint and coating while at the same time providing for a smooth paint or coating surface.

BACKGROUND OF THE INVENTION

The present invention is directed to modified calcium carbonate containing minerals and their use, e.g., in paints and coatings at low dosage, and more particularly to a modified calcium carbonate containing mineral providing a matting effect in paints and coatings, while at the same time providing a smooth paint or coating film.

Today matting effects can be achieved by different means as long as they provide for a micro-roughness of the paint or coating film surface, wherein the incident light is scattered in a way that results in matted surface. The physics behind this is known. The conditions to obtain a perfectly matted effect without resorting to complete light absorption are to scatter the incoming light away from the specular reflectance angle. This means diffracting the directed light that illuminates the substrate causing diffuse scatter.

In paint and coating industries a variety of such matting agents are known and mixed into the paints and coatings such as silica, waxes, organic materials and even fillers are added to form a micro-rough surface after the drying process of the paint or coating. It is recognized as a general rule that the higher the dosing of the matting agent in a paint or coating, the stronger the matting effect. In contrast, products with larger particle sizes are stronger in matting efficiency but the resulting paint or coating film surface is not so smooth. Matting agents with smaller average particle size distribution do not provide sufficient matting effect, but provide for a smoother paint or coating surface.

Japanese patent application JP-A-2003 147275 discloses a coating material composition comprising a binder component and a calcium carbonate treated with phosphoric acid. Said coating material provides for a matte surface providing that the treated calcium carbonate has a mean particle diameter of less than 10 μm, a BET specific surface area of 70-100 m²/g and oil absorption of 130-20 ml/100 g.

WO 2006/105189 A1 refers to aggregated particulate minerals and compositions comprising aggregated calcium carbonate. Said dry aggregated calcium carbonate beads have a weight median aggregate particle size d₅₀ of at least 5 μm and even a size of at least 100 μm. Said aggregated calcium carbonate beads are made into paper, paint, coatings or ceramics.

U.S. Pat. No. 5,634,968 refers to carbonate containing mineral fillers, more particularly for use as matting agents. Said mineral materials are natural and/or precipitated calcium carbonates with a d₅₀ of 9.6-20.5 μm, wherein ground natural calcium carbonates are preferred.

U.S. Pat. No. 5,531,821 and U.S. Pat. No. 5,584,923 disclose and claim acid resistant calcium carbonates made by mixing calcium carbonate with anionic salts and cationic salts. Said acid resistant calcium carbonate is being used in neutral to weakly acid paper making processes.

U.S. Pat. No. 6,666,953 disclose a pigment of filler containing natural carbonate which is treated with one or more providers of medium-strong to strong H₃O⁺ ions and gaseous CO₂.

US 2008/0022901 refers to mineral pigments containing a dry product formed in situ by the multiple reaction between a calcium carbonate and the product or products of reaction of said carbonate with one or more moderately strong to strong H₃O⁺ ion donators and the product or products of reaction of the said calcium carbonate with gaseous CO₂ formed in situ and/or originating from an external supply and one or more compounds of formula R-X.

EP 2 264 109 A1 and EP 2 264 108 A1 disclose a process for preparing a surface-reacted calcium carbonate and its use as well as a process to prepare a surface-reacted calcium carbonate implementing the a weak acid, resulting products and uses thereof.

DESCRIPTION OF THE INVENTION

The present invention relates to a surface modified calcium carbonate comprising mineral of spherical shape and its use. Particularly they are used in paints or coatings, providing for a matting effect and a smooth paint or coating surface at low dosage. Surface modified calcium carbonate comprising mineral can be obtained by processes as described in the prior art.

The process for producing the spherical shaped surface modified calcium carbonate comprising particles basically comprises the steps of:

-   -   a) providing at least one aqueous calcium carbonate containing         mineral slurry;     -   b) providing at least one water-soluble acid;     -   c) optionally providing further gaseous CO₂ via an external         route;     -   d) contacting said aqueous calcium carbonate containing mineral         slurry of step a) with said acid of step b) and with said CO₂         generated in situ and/or supplied externally of step c) under         stirring conditions.     -   e) optionally dewatering the aqueous slurry;

The spherical ball-shaped surface modified calcium carbonate containing mineral obtained by such a process has specific BET surface area of more than 15 m²/g, and preferably from about 20 m²/g to about 200 m²/g, more preferably from about 30 m²/g to about 150 m²/g, even more preferably from about 40 m²/g to about 100 m²/g, and a median grain diameter of from about from 4 μm to about 100 μm, preferably from about 5 μm to about 75 μm, more preferably from about 1 μm to about 50 μm, still more preferably from about 15 μm to about 30 μm.

Within the context of the present invention, spherical shaped means that the appearance of the spherical shaped surface modified calcium carbonate containing mineral is globular or ball-shaped.

Further, the ball-shaped surface modified calcium carbonate comprising mineral has a narrow particle size distribution d₉₈/d₅₀ preferably smaller than 3, more preferably smaller than 2.5, preferably in a range from 1.4 to 2.9.

These specific properties are particularly obtained when the at least one aqueous calcium carbonate containing mineral slurry has a solids content of greater than 4 wt.-%, preferably from 5 wt.-% to about 20 wt.-%, more preferably from about 7 wt. % to about 10 wt. %, based on the weight of the slurry. Lower solids content such as below 4 wt. % or higher solids content such as above 20 wt. % could be used as well.

It is preferred that the calcium carbonate containing mineral is selected from the group consisting of marble, chalk, limestone, calcite, dolomite and precipitated calcium carbonate (PCC), and mixtures thereof. PCC is sometime also referred to as synthetic calcium carbonate. Said calcium carbonate containing mineral are basically supplied in an aqueous system. Said aqueous system frequently being a slurry.

A “slurry” in the meaning of the present invention is a suspension (comprising essentially insoluble solids and water and optionally further additives) and usually has a higher density than the liquid without solids from which it is formed.

In accordance with the present invention the calcium carbonate containing mineral comprises generally either ground marble, chalk, limestone, calcite, dolomite or mixtures thereof, but encompasses also precipitated calcium carbonate (PCC), which is also known as synthetic calcium carbonate as starting material.

Advantageously, the calcium carbonate containing mineral present in the slurry has a weight median particle diameter of 0.01 μm to 10 μm, preferably from 0.2 μm to 2 μm, more preferably from 0.5 μm to 1 μm, as measured by Sedigraph.

The slurry in the process of the present invention can be stabilized or not. In case the slurry is stabilized, conventional dispersants known to the skilled person can be used. A preferred dispersant is polyacrylic acid, or other partially or fully neutralized forms like sodium polyacrylate.

The at least one water-soluble acid provided in step b) can be chosen from acids as disclosed in EP 2 264 109 A1 and/or EP 2 264 108 A1. An example of such acid is phosphoric acid or sulfuric acid.

The at least one water-soluble acid provided in step b) is dosed in a molar amount relative to the numbers of moles of the calcium carbonate containing mineral in the range from 0.01 mol/mol to about 1 mol/mol. Preferably from 0.1 to 0.6 mol/mol.

The at least one water-soluble acid provided in step b) is added in one or more steps, preferably in one step to said aqueous calcium carbonate containing mineral slurry.

The addition in one or more steps can also be executed over a defined time period, such as a time period of 5 min, 10 min, 20 min, 30 min, 60 min or longer such as 120 min or 180 min. When the addition is made over a certain time period, the addition can be made continuously or discontinuously. E.g. adding 30% dose over 10 minutes would therefore be 3% dose per minute.

Optionally, after the addition of at least one water-soluble acid in step b) gaseous CO₂ can be provided into the slurry by an external route. Such addition of gaseous CO₂ is known to the skilled person. However CO₂ can be generated also in situ when the carbonate containing mineral is reacting with the H₃O+ ions. Therefore the origin of CO₂ can be either in situ or supplied externally or even a combination thereof.

Irrespective of the optional addition of gaseous CO₂ to the slurry, the process of the present invention further comprises the step e) which is contacting said aqueous calcium carbonate containing mineral slurry of step a) with said acid of step b) and with said in situ generated and/or externally supplied CO₂ of step d), said contacting being made under stirring or mixing conditions.

The process may further comprise step c) wherein a processing agent is provided before, during or after the at least one aqueous calcium carbonate containing mineral slurry of step a) is contacted with said at least one water-soluble acid of step b) and with said in situ generated or externally supplied CO₂ or a combination thereof of step d).

Preferably the processing agent is provided before and/or during the at least one aqueous calcium carbonate containing mineral slurry of step a) is contacted with said at least one water-soluble acid of step b) and with said in situ generated or externally supplied CO₂ or a combination thereof of step d).

Thus, in a particular embodiment of the process as herein disclosed, the processing agent is added before the addition of the water-soluble acid of step b), followed by the steps c), d), and e)

In a further particular embodiment of the process as herein disclosed, the processing agent is added during the addition of the water-soluble acid of step b), followed by the steps c), d), and e).

And still in another embodiment of the process as herein disclosed, the processing agent is added after the addition of the water-soluble acid of step b), followed by the steps c), d), and e).

Preferably, the processing agent is added before and/or during the addition of the water-soluble acid of step b), followed by the steps c), d), and e).

Said at least one processing agent may be preferably selected from the group comprising ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, aluminum sulfate, and/or their hydrated forms, silicates, water-soluble cationic polymers, water-soluble amphoteric polymers, water-soluble non-ionic polymers and combinations thereof, as well as seeds of precipitated calcium carbonate (PCC), surface reacted calcium carbonate of the prior art or ball-shaped surface reacted calcium carbonate comprising mineral obtained by the process of the present invention.

Without being bound to any theory the inventors believe that the at least one processing agent functions as a coagulating agent promoting the assembly of calcium carbonate containing mineral particles which upon further exposure to the mentioned chemicals in the process provide for the ball-shaped surface modified calcium carbonate containing particles.

The reaction conditions of the process as herein described take place in an aqueous environment in a temperature range above and including 10° C., such as from about 25° C. to about 95° C., preferably in a range from about 30° C. to about 80° C., more preferably from about 50° C. to about 75° C.

The processing agent is dosed in one or more steps, preferably in one step to said aqueous calcium carbonate containing mineral slurry before, during or after the addition of the at least one water-soluble acid of step b). The processing agent is added in amounts of up to 8 wt. % based on the dry weight of the slurry. Preferably the processing agent is added in amounts from 0.01 wt. % to about 5 wt. %, more preferably from about 0.05 wt. % to about 4 wt. %, still more preferably from about 0.4 wt % to about 3 wt. %, based on the dry weight of the slurry.

In a particular embodiment, the processing agent is aluminum sulfate. In a further particular embodiment, the processing agent is aluminum sulfate in its hydrated form. In preferred embodiment the processing agent is aluminum sulfate hexadecahydrate.

In still a further embodiment, the content of the aluminum sulfate in the process as herein described is up to 4 wt. % based on the weight of the dry slurry, preferably in the range from about 0.1 wt % to about 2 wt. %, more preferably from about 0.2 wt % to about 1 wt %, based on the dry slurry. It has to be considered that the content of aluminum sulfate is crucial thus dosing of the hydrate requires the corresponding adaption to achieve the desired amount.

After mixing the slurry can be dewatered in an optional step e), by any type of thermal and/or mechanical methods known to the skilled person.

The aqueous slurry thus obtained can be further treated such as drying the aqueous slurry in order to obtain a dry surface modified calcium carbonate comprising mineral of spherical or ball-shape. The drying method applied to obtain a dry surface modified calcium carbonate comprising mineral of spherical or ball-shape can be any kind of drying method well known to the skilled person.

The aqueous ball-shaped surface modified calcium carbonate containing mineral comprised in the slurry obtained by the process of the present invention, has a median grain diameter of from 4 μm to about 100 μm, preferably from about 5 μm to about 75 μm, more preferably from about 10 μm to about 50 μm, still more preferably from about 15 μm to about 30 μm.

The aqueous ball-shaped surface modified calcium carbonate containing mineral comprised in the slurry obtained by the process of the present invention, has at the same time a narrow particle size distribution d₉₈/d₅₀ preferably smaller than 3, more preferably smaller than 2.5, preferably in a range from 1.4 to 2.9.

Still further the aqueous surface modified calcium carbonate containing mineral comprised in the slurry obtained by the process herein described, has a BET specific surface area of more than 15 m²/g, and preferably from about 20 m²/g to about 200 m²/g, more preferably from about 30 m²/g to about 150 m²/g, even more preferably from about 40 m²/g to about 100 m²/g.

A BET specific surface area within the meaning of the present invention relates to the specific surface area measured via the method provided in the examples section hereafter.

In a preferred embodiment, the aqueous ball-shaped surface modified calcium carbonate containing mineral comprised in the slurry obtained by the process of the present invention, has a BET specific surface area from 30 m²/g to about 90 m²/g and a median grain diameter from 10 μm to 50 μm.

According to another embodiment, the aqueous slurry of the ball-shaped surface modified calcium carbonate containing mineral obtained by the process as herein described can be further dried to obtain a dry ball-shaped surface modified calcium carbonate containing mineral.

According to still another embodiment, said dry ball-shaped surface modified calcium carbonate containing mineral, has a BET specific surface area of more than 15 m²/g, and preferably from about 20 m²/g to about 200 m²/g, more preferably from about 30 m²/g to about 150 m²/g, even more preferably from about 40 m²/g to about 100 m²/g. Further said dry ball-shaped surface modified calcium carbonate containing mineral, has a median grain diameter of from 4 μm to about 100 μm, preferably from about 5 μm to about 75 μm, more preferably from about 10 μm to about 50 μm, still more preferably from about 15 μm to about 30 μm.

FIG. 1: shows particle size distribution curves of surface modified calcium carbonate containing minerals according to the present invention

FIG. 2 a: shows an SEM picture of ball-shaped surface modified calcium carbonate containing mineral of E2, at 500× magnification.

FIG. 2 b: shows an SEM picture of ball-shaped surface modified calcium carbonate containing mineral of E2, at 2500× magnification.

FIG. 3 a: shows an SEM picture of ball-shaped surface modified calcium carbonate containing mineral of E6, at 500× magnification.

FIG. 3 b: shows an SEM picture of ball-shaped surface modified calcium carbonate containing mineral of E6, at 2 500× magnification.

USE OF THE INVENTION

The inventive ball-shaped surface modified calcium carbonate containing mineral or a slurry comprising said inventive ball-shaped surface modified calcium carbonate containing mineral may be used in paper, tissue paper, digital photo paper, paints, coatings, adhesives, plastics, or in waste water treatment or waste water treating agents.

In preferred embodiment, the inventive ball-shaped surface modified calcium carbonate containing mineral is used in paints or coatings as a matting agent. By matting agent, the applicant understands an agent being capable of scattering the incoming light away from the specular reflectance angle. This means diffracting the directed light that illuminates the substrate causing diffuse scatter.

In particular the matting agent is present in amounts of 1 to 10 wt. %, preferably in amounts of 2 to 7 wt. %, more preferably in amount of 3 to 5 wt. %, based on the total weight of the paint or coating.

The paints or coatings comprising the inventive ball-shaped surface modified calcium carbonate containing mineral as matting agents in the amounts mentioned above provide for a surface gloss at 85° in the range of below 10 gloss units (GU), preferably from 0.5 to 9.5, more preferably from 1 to 8, still more preferably from 2 to 6.5, of the dried film of the paint or coating, when measured according to DIN 67 530, which is very surprising due to the low content of the matting agent.

A further advantage of the inventive ball-shaped surface modified calcium carbonate containing mineral present in amounts as described above is, that besides the matting effect of the dried film of the paint or coating, the surface of said dried paint or coating is smooth.

Thus, the inventive ball-shaped surface modified calcium carbonate containing mineral of the present invention when used in paints and/or coatings, provide for matt appearance while at the same time providing a haptic smooth surface.

The following examples are meant to illustrate the invention without restricting its scope:

Measurement Methods

The following measurement methods are used to evaluate the parameters given in the description, examples and claims.

BET Specific Surface Area (SSA) of a Material

The specific surface area is measured with nitrogen via the BET method according to ISO 9277 using a Gemini V sold by the company MICROMERITICS™, following conditioning of the sample by heating at 250° C. for a period of 30 minutes. Prior to such measurements, the sample is filtered within a Buchner funnel, rinsed with deionised water and dried overnight at 90° C. to 100° C. in an oven. Subsequently, the dry cake is ground thoroughly in a mortar and the resulting powder placed in a moisture balance at 130° C. until a constant weight is reached.

Solids Content of an Aqueous Slurry

The slurry solids content (also known as “dry weight”) is determined using a Moisture Analyser HR73 commercially available from Mettler-Toledo with the following settings: temperature of 120° C., automatic switch off 3, standard drying, 5-20 g of slurry.

Particle Size Distribution (Mass % Particles with a Diameter<x) and Weight Median Diameter (d50) of Non-Surface Reacted Calcium Carbonate Comprising Material (i.e. Calcium Carbonate Starting Material)

Weight median grain diameter and grain diameter mass distribution of a particulate material such as calcium carbonate, are determined via the sedimentation method, i.e. an analysis of sedimentation behavior in a gravimetric field. The measurement is made with a Sedigraph™ 5120.

The method and instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurements is carried out in an aqueous solution of 0.1 wt-% Na₄P₂O₇. The samples were dispersed using a high speed mixer and ultrasound.

Median Grain Diameter d₅₀ and d₉₈ of Ball-Shaped Surface Modified Calcium Carbonate Containing Mineral

Median grain diameter, d₅₀ and d₉₈, of ball-shaped surface modified calcium carbonate containing mineral was determined using a Malvern Mastersizer 2000 Laser Diffraction System, with a defined RI of 1.57 and iRI of 0.005, Malvern Application Software 5.60. The measurement was performed on an aqueous dispersion. The samples were dispersed using a high-speed stirrer. In this respect, the d₅₀ and d₉₈ values define the diameters, at which 50 vol.% or 98 vol.% of the particles measured, have a diameter smaller than d₅₀ or d₉₈ value, respectively.

Viscosity Measurements

A. ICI Viscosity According to EN ISO 2884-1

The ICI viscosity was measured according to EN ISO 2884-1 using a cone- and plate viscometer (Epprecht Instruments+Controls, Bassersdorf, Switzerland) at a shear rate of 10 000 l/s at a temperature of (23±0.2° C. The measured value after 15 s, which should be a constant value, depicts the measured viscosity of the sample.

B. Viscosity with a Paar Physica M301 PP25 Rheometer

This measurement was conducted with a Paar Physica M301 PP25 Rheometer, from the company Anton Paar GmbH, Austria, according to the following regime:

Temp.: 23° C.

Starting shear rate: 0.1 l/s End shear rate: 100 l/s, with a logarithmic gradient of 10 measurement points per decade, and each measurement point taken after 5 seconds.

The measurement points are displayed in a decimal logarithmic manner, so that a linear plot with a negative slope results from this measurement. The x-axis of the graph represents the shear rate in a decimal logarithmic manner, and the y-axis depicts the measured viscosity in Pa·s.

Gloss of a Coated Surface

The Gloss values are measured at the listed angles according to DIN 67 530 on painted surfaces prepared with a coater gap of 150 μm and 300 μm on contrast cards. The contrast cards used are Leneta contrast cards, form 3-B-H, size 7⅝×11⅜ (194×289 mm), sold by the company Leneta, and distributed by Novamart, Stäfa, Switzerland. The gloss is measured with a gloss measurement device from the company Byk Gardner, Geretsried, Germany. The gloss is obtained by measuring 5 different points on the card with the gloss measurement device, and the average value is calculated by the device and can be derived from the display of the device.

Scrubbing Test

For the scrubbing test, Laneta contrast cards, form 3-B-H, size 7⅝×11⅜ (194×289 mm), sold by the company Leneta, and distributed by Novamart, Stäfa, Switzerland, coated with a coater gap of 300 μm, as mentioned above were submitted to a scrubbing test. The scrubbing was performed with a square felt-glider 22 mm×22 mm fixed on the scrubbing device. The arm holding the felt-glider was loaded with a 500 g weight pressing the felt-glider onto the surface of the coated Leneta card. The scrubbing path length was 5 cm and 50 cycles (1 clyle=forth and back) where performed within 60 sec±2 sec. The felt-glider was of the fix-o-moll type, provided by from Wilhelm Ritterath GmbH, Meckenheim, Germany. Gloss was measured according to DIN 67 530 at 60° and 85° before and after scrubbing.

Determination of Colour Values (Rx, Rv, Rz)

The colour values Rx, Ry, Rz are determined over the white and black fields of the Leneta contrast card, and are measured with a spectraflas SF 450 X spectrophotomer of the company Datacolor, Montreuil, France.

Contrast Ratio (Opacity) of a Coated Surface

Contrast ratio values are determined according to ISO 2814 at a spreading rate of 7.5m²/l.

The contrast ratio is calculated as described by the equation below:

${{Contrast}\mspace{14mu} {{ratio}\lbrack\%\rbrack}} = {\frac{{Ry}_{black}}{{Ry}_{white}} \times 100\%}$

with Ry_(black) and Ry_(white) being obtained by the measurement of the color values.

EXAMPLES

The following illustrative examples of the invention involve contacting a calcium carbonate containing mineral, namely a ground natural marble according to the process of the present invention in a jacketed steel reactor equipped with a laminar mixing system in a batch size of 10 l. The solid content is adjusted to either 10 wt. % or 8 wt. % solids, as indicated in table 1.

The addition of the at least one water-soluble acid of step b) and the contacting of said aqueous calcium carbonate containing mineral slurry of step a) with said acid of step b) and with said CO₂ generated in situ and/or supplied externally of step d) take place in a stirred reactor under stirring conditions such as to develop an essentially laminar flow.

The marble used in the process of the present invention and indicated as H90 in table 1., is a commercially available product from the applicant sold under the brand name Hydrocarb® 90-ME 78%, which is a natural ground marble from Molde in Norway, having a top cut d₉₈ of 5 μm, and weight median particle size d₅₀ of 0.7 μm (size determined by Sedigraph), and provided in form of a slurry with solids content of 78 wt. % based on dry matter of the slurry and a viscosity of 400 mPas.

The mixing speed is adjusted to either 140 or 300 rpm, and the temperature is adjusted to 70° C. Prior to the dosing of a 30 wt. % of phosphoric acid solution, which is added over a period of 10 or 30 min, the processing agent aluminum sulfate hexadecahydrate was added at once to the calcium carbonate containing mineral slurry in amounts of 0.5 wt. % or 0.6 wt. %.

The reaction was mixed at the indicated mixing speeds and times according to table 1.

TABLE 1 Tank Conditions H₃PO₄ 30 wt. % Target Added Al₂(SO₄)₃*16H₂O Batch Mixing Slurry Feed dosing over a Final Size Speed Solids Slurry Temp. of period conc. Addition (1) (rpm) wt. % Type ° C. wt. % of wt. % time E1 10.00 300 10.0 H90 70 30.0 10 min 0.5 b/a E2 10.00 300 8.0 H90 70 30.0 10 min 0.5 b/a E3 10.00 300 10.0 H90 70 30.0 10 min 0.6 b/a E4 10.00 140 10.0 H90 70 30.0 10 min 0.5 b/a E5 10.00 140 8.0 H90 70 30.0 10 min 0.5 b/a E6 10.00 140 10.0 H90 70 30.0 30 min 0.5 b/a E7 10.00 140 10.0 H90 70 30.0 10 min 0.6 b/a b/a means addition before acid

The particle size distribution (PSD) of examples E1 to E7 were measured on a Malvern Mastersizer and the particle size distribution curves are shown in FIG. 1. Table 2 provides for the BET specific surface area SSA, the top cut d₉₈ and the median grain diameter d₅₀ for the examples E1 to E/.

TABLE 2 E1 E1 E3 E4 E5 E6 E7 SSA m²/g 53.9 62.5 56.7 62.2 56.9 57.8 59.6 d₅₀* μm 19.7 27.9 18.9 34.5 30.4 15.3 22.8 d₉₈* μm 41.6 46.6 38.3 57.8 50.7 27.3 39.3 d₉₈/d₅₀ 2.1 1.7 2.1 1.7 1.7 1.8 1.7 *determined by Malvern Mastersizer

The examples E2, E6, and E7 of surface modified calcium carbonates were tested in paints. For this, E2, E6 and E7 were mixed in formulations and compared with matting agents used in this area such as diatomaceous earths (C1, C3) or aluminum silicate (C2). The dosage level of all matting agents was at 3 wt. %. Said formulations further comprise common additives such as defoamers, dispersing agents, sodium hydroxide, fungicides, bactericides, titanium dioxide (rutile), talcum, fillers, pigments, thickeners, plasticizer, viscosity modifiers, water, and others known to the skilled person. Table 3 provides for the composition of the test paint.

TABLE 3 Base test paint C1 C2 C3 P2 P6 P7 Water g 331  331  331  331  331  331  Calgon N neu g 1 1 1 1 1 1 Bermocoll EHM 200 g 3 3 3 3 3 3 Sodium hydroxide, 10% g 2 2 2 2 2 2 BYK 038 g 3 3 3 3 3 3 ECODIS P 50 g 3 3 3 3 3 3 Mergal 723K g 2 2 2 2 2 2 Tiona 595 g 60  60  60  60  60  60  Finntalc M20SL - AW g 80  80  80  80  80  80  Omyacarb EXTRA - CL g 150  150  150  150  150  150  Omyacarb 2 - GU g 220  220  220  220  220  220  Mowilith LDM 6119, 50% g 115  115  115  115  115  115  Matting agents Celite 281 g 30  (Diatomaceous Earth) OpTiMat 2550 g 30  (Aluminum Silicate) Celatom MW 27 g 30  (Diatomaceous Earth) E2 g 30  E6 g 30  E7 30  COAPUR 4435* g 10  10  10  10  10  10  (Polyurethan thickener) Total g 1 010    1 010    1 010    1 010    1 010    1 010    *the 1% of COAPUR 4435 was added after the paint production to raise the viscosity to an applicable level.

The fillers and pigments were replaced on a volume basis, i.e. at identical Pigment Volume Concentration (PVC). The paints were tested for dry opacity (ISO 2814), whiteness Ry (DIN 53145) and sheen (DIN67530) (gloss at 85°). The components and functions of the materials for the base test paint are commercially available products known to the skilled person and listed in table 4 hereto below.

TABLE 4 Material for base paint. Producer Chemical basis Function Base test paint Water In house, deionized H₂O Solvent Calgon N new BK Giulini Chemie Sodium polyphosphate Wetting and dispersing agent Bermocoll EHM 200 AkzoNobel Corp. Ethyl Hydroxyethyl cellulose Thickener Sodium hydroxide, 10% Various NaOH solution pH regulator BYK 038 Byk Chemie Mineral oil basis Defoamer ECODIS P 50 Coatex SA Sodium salt of acrylic polymer Wetting and dispersing agent Mergal 723K Troy Chemie GmbH Benzisothiazolone basis, without Preservative formaldehyde Tiona 595 Millenium Inorganic Chemials Titanium dioxide, rutile (Al, org. White pigment coated) Finntalc M20SL - AW Mondo Minerals Magnesium Silicate Extender Omyacarb EXTRA - CL Omya Clariana SA Calcium carbonate, marble Extender Omyacarb 2 - GU Omya Calcium carboanate, marble Extender Mowilith LDM 6119, 50% Clariant non-plasticized aqueous polymer Binder (copolymer) dispersion based on styrene and an acrylic acid ester Matting agents Celite 281 (Diatomaceous Earth) Celite France Diatomaceous Earth (silica) Matting agent OpTiMat 2550 Imerys Aluminum silicate Matting agent Celatom MW 27 EP Minerals Diatomaceous Earth Matting agent E2 Omya Surface modified marble Matting agent E6 Omya Surface modified marble Matting agent Paint thickener Various Producers Polyurethane Thickener

The performance of the tested paints is summarized in table 5, wherein C1, C2, and C3 refer to comparative paint examples and P2, P6, and P7 refer to the paint examples comprising the surface modified calcium carbonate containing minerals obtained by the process of the present invention.

TABLE 5 Performance of paints C1 C2 C3 P2 P6 P7 Temperature ° C. 23   23   23   23   23   23   ICI Viscosity 180    170    160    200    200    200    Paar Physica Viscosity Viscosity, γ = 1 s⁻¹ Pas 16 800    19 000    18 700    22 500    24 100    24 600    Viscosity, γ = 5 s⁻¹ Pas 6 450    7 130    7 010    8 940    9 680    9 240    Viscosity, γ = 10 s⁻¹ Pas 4 380    4 790    4 700    6 150    6 640    6 190    Viscosity, γ = 40 s⁻¹ Pas 2 080    2 270    2 160    2 860    3 60   2 710    Optical properties 150 micrometer, 300 micrometers and 7.5 m²/l 150 micrometer Ry % 90.4  89.8  90.2  90.9  91.0  91.1  Ry over black % 87.9  87.4  88.0  89.1  89.1  89.4  Yellowness Index 2.5 2.6 2.5 2.0 2.0 2.1 Contrast Ratio % 97.2  97.4  97.5  98.0  98.0  98.2  Gloss 85° 4.9 2.9 3.9 3.6 5.6 3.6 300 micrometer Ry % 90.9  90.2  90.8  91.6  91.7  91.8  Ry over black % 90.3  89.7  90.3  91.2  91.3  91.3  Yellowness Index 2.7 2.8 2.7 2.3 2.3 2.4 Contrast Ratio % 99.3  99.4  99.4  99.5  99.5  99.5  Gloss 85° 5.0 3.3 4.5 4.4 6.2 4.1 Opacity at 7.5 m²/l Contrast ratio % 98.1  98.1  98.1  98.8  98.8  98.9  Polishing CROWN 300μ Gloss 60° unscrub 2.7 2.6 2.6 2.7 2.7 2.7 Gloss 60° scrub 5.7 5.9 5.6 6.6 6.5 6.5 Gloss 85° unscrub 5.4 3.1 4.6 4.2 6.2 4.2 Gloss 85° scrub 29.5  18.7  27.1  31.7  36.3  29.5 

As can be seen from the results of table 4, the examples of the present invention show a performance in matting effect similar to matting agents of the prior art. Such matting effects have up to now not been observed for paints comprising the surface modified calcium carbonate containing minerals having a median grain diameter from about 10 μm to about 50 μm, and having a BET specific surface area from about 30 m²/g to about 90 m²/g of the present invention. The paint properties were measured for dry opacity at 150 μm and 300 μm film thickness, and the contrast ratio was determined at 7.5 m²/l. P2, P6, and P7 provide for a gloss at 85° of 4.1, 6.2, and 4.1 at a coating thickness of 300 μm. This is also in expectation of the matting effect to decrease with decreasing particle size.

Therefore, the present invention provides for alternative matting agents based on calcium carbonate containing mineral, providing a matting effect when used in paints and/or coatings, while at the same time providing a haptic smooth surface. Further to this, the present invention provides for a process for obtaining such surface modified calcium carbonate containing minerals, wherein said surface modified calcium carbonate containing minerals can be used in paper and paper coating, tissue paper, digital photo paper, paints, coatings, adhesives, plastics, or in waste water treating agents. 

1. Process for producing ball-shaped spherical surface modified calcium carbonate comprising particles comprising the steps of: a) providing at least one aqueous calcium carbonate containing mineral slurry; b) providing at least one water-soluble acid; c) optionally providing further gaseous CO₂ via an external route; d) contacting said aqueous calcium carbonate containing mineral slurry of step a) with said acid of step b) and with said CO₂ generated in situ and/or supplied externally of step c) under stirring conditions. e) optionally dewatering the aqueous slurry;
 2. Process according to claim 1, wherein at least one processing agent is added before, during or after said aqueous calcium carbonate containing mineral slurry of step a) is contacted with said acid of step b) and with said CO₂ generated in situ and/or supplied externally of step d).
 3. Process according to claim 1 or 2, wherein said calcium carbonate containing mineral is selected from the group consisting of marble, chalk, limestone, calcite, dolomite, precipitated calcium carbonate (PCC), and mixtures thereof.
 4. Process according to claim 1, wherein said calcium carbonate containing mineral in the slurry has a weight median particle diameter of 0.01 μm to 10 μm, preferably from 0.2 μm to 2 μm, more preferably from 0.5 μm to 1 μm.
 5. Process according to claim 1, wherein said slurry of step a) has a solids content of greater than 4 wt.-%, preferably from 5 wt.-% to about 20 wt.-%, based on the weight of the slurry.
 6. Process according to claim 1, wherein said slurry of step a) is stabilized by the addition of one or more dispersants.
 7. Process according to claim 1, wherein the at least one water-soluble acid of step b) has a pK_(a) from 0 to 6, preferably has a pK_(a) from 0 to 2.5.
 8. Process according to claim 1, wherein the at least one water-soluble acid of step b) is selected from phosphoric acid, citric acid, boric acid, or mixtures thereof.
 9. Process according to claim 1, wherein the at least one water-soluble acid is dosed in a molar amount relative to the numbers of moles of the calcium carbonate containing mineral in the range from 0.01 mol/mol to about 1 mol/mol, preferably from 0.1 to 0.6 mol/mol.
 10. Process according to claim 1, wherein the at least one water-soluble acid provided in step b) is added in one or more steps, preferably in one step to said aqueous calcium carbonate containing mineral slurry.
 11. Process according to claim 1, wherein the addition of the at least one water-soluble acid of step b) and the contacting of said aqueous calcium carbonate containing mineral slurry of step a) with said acid of step b) and with said CO₂ generated in situ and/or supplied externally of step d) take place in a stirred reactor under stirring conditions such as to develop an essentially laminar flow.
 12. Process according to claim 1, wherein the process takes place in an aqueous environments in a temperature range above and including 20° C., such as from about 25° C. to about 95° C., preferably in a range from about 30° C. to about 80° C., more preferably from about 50° C. to about 75° C.
 13. Process according to claim 2, wherein the processing agent is added in amounts of up to 8 wt. % based on the dry weight of the slurry, preferably in amounts from 0.01 wt. % to about 5 wt. %, more preferably from about 0.05 wt. % to about 4 wt. %, still more preferably from about 0.4 wt % to about 3 wt. %, based on the dry weight of the slurry.
 14. Process according to claim 2, wherein the at least one processing agent is selected from the group comprising ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, aluminum sulfate, and/or their hydrated forms, silicates, water-soluble cationic polymers, water-soluble amphoteric polymers, water-soluble non-ionic polymers and combinations thereof, as well as seeds of precipitated calcium carbonate (PCC), or surface reacted calcium carbonate, or ball-shaped surface reacted calcium carbonate comprising mineral.
 15. Process according to claim 14, wherein the processing agent, preferably aluminum sulfate or its hydrate, more preferably aluminum sulfate hexadecahydrate is dosed in one or more steps, preferably in one step to said aqueous calcium carbonate containing mineral slurry.
 16. Process according to claim 15, wherein the content of aluminum sulfate is up to 5 wt. % based on the weight of the dry slurry.
 17. Process according to claim 1, wherein the processing agent is added before, during or after step b).
 18. Aqueous slurry of ball-shaped surface modified calcium carbonate containing mineral obtained according to the process of claim
 1. 19. Aqueous slurry according to claim 18, wherein said surface modified calcium carbonate containing mineral has a median grain diameter of from 4 μm to about 100 μm, preferably from about 5 μm to about 75 μm, more preferably from about 10 μm to about 50 μm, still more preferably from about 15 μm to about 30 μm, determined by Malvern Mastersizer.
 20. Aqueous slurry according to claim 18, wherein said surface modified calcium carbonate containing mineral has a BET specific surface area of more than 15 m²/g, and preferably from about 20 m²/g to about 200 m²/g, more preferably from about 30 m²/g to about 150 m²/g, even more preferably from about 40 m²/g to about 100 m²/g.
 21. Aqueous slurry according to claim 20, wherein said surface modified calcium carbonate containing mineral has a BET specific surface area from 30 m²/g to about 90 m²/g and a median grain diameter from 10 μm to 50 μm, determined by Malvern Mastersizer.
 22. Aqueous slurry according to claim 18, wherein the ball-shaped surface modified calcium carbonate comprising mineral has a narrow particle size distribution d₉₈/d₅₀ preferably smaller than 3, more preferably smaller than 2.5, preferably in a range from 1.4 to 2.9.
 23. Surface modified calcium carbonate containing mineral obtained by drying the aqueous slurry of claim
 18. 24. Surface modified calcium carbonate containing mineral of claim 23, wherein the surface modified calcium carbonate containing mineral has a BET specific surface area of more than 15 m²/g, and preferably from about 20 m²/g to about 200 m²/g, more preferably from about 30 m²/g to about 150 m²/g, even more preferably from about 40 m²/g to about 100 m²/g.
 25. Surface modified calcium carbonate containing mineral of claim 23, wherein the surface modified calcium carbonate containing mineral has a median grain diameter of from 4 μm to about 100 μm, preferably from about 5 μm to about 75 μm, more preferably from about 10 μm to about 50 μm, still more preferably from about 15 μm to about 30 μm, determined by Malvern Mastersizer.
 26. Surface modified calcium carbonate containing mineral of claim 23, wherein the surface modified calcium carbonate containing mineral has a BET specific surface area from 30 m²/g to about 90 m²/g and a median grain diameter from 10 μm to 50 μm, determined by Malvern Mastersizer.
 27. Paint or coating comprising the surface modified calcium carbonate containing mineral of the aqueous slurry of claim 1, or dried version thereof.
 28. The paint or coating according to claim 27, wherein the surface modified calcium carbonate or dried version thereof is used as a matting agent in paints and coatings.
 29. The paint or coating according to claim 28, wherein the matting agent is present in amounts of 1 to 10 wt. %, preferably of 2 to 7 wt. %, more preferably of 3 to 5 wt. % based on the wet paint.
 30. The paint or coating according to claim 29, wherein surface of the dried paint or coating has a gloss at 85° in the range of below 10, preferably from 0.5-9.5, more preferably from 1 to 8, still more preferably from 2-6.5, when measured according to DIN 67
 530. 31. Paper, tissue paper, digital photo paper, paints, coatings, adhesives, plastics, or waste water treating agent comprising the ball-shaped surface modified calcium carbonate containing mineral of the aqueous slurry of claim 18, or a dried version thereof. 